Within my example, Flygon would be running Choice Scarf vs Noivern's Choice Specs. Flygon would be faster. It's U-Turn would hit much harder. And the only moves it has to hit harder with is Draco Meteor on certain Pokemon, and maybe Hurricane, which is very unreliable. Boomburst doesn't receive STAB. It's utility with Specs is limited to Switcheroo and U-Turn, and I've already covered the problem with U-Turn.
Noivern's attacks are really weak comparatively, even with Choice Specs, so I don't think it can really afford to go without one.
Out of pure curiosity, does anyone have any calcs for comparison of these?
Say, Choice specs noivern vs scarf flygon? (Because Flygon isn't outrunning much without it, and noivern isn't hitting that hard without specs. Noivern also doesn't need scarf to outrun much, so that calc is pretty moot.) I'm aware this is very skewed, but I don't see the point to using choice scarf flygon over noivern at all, other than better typing for reducing residual damage.
You're not outspeeding much notable targets that spec noivern wouldn't have, with the first thing to hit my mind being Talonflame (who has priority brave bird and certainly wouldn't flare blitz you), second being jolteon (I'll give you that one). Pulling off of Serebii's charts, the ONLY other notable one i can really see is Weavile, which will murder both flygon/noivern regardless with ice shard. Meanwhile, Noivern hits much harder, and outspeeds nearly the same things. This is all, of course, assuming choice sets, with scarfgon vs specs Noivern.
I guess specs Noivern can't outspeed scarfed version of other certain pokes, so there's Scarf Genesect, I guess. But this is also why Frisk is so useful, because I can actually see things like surprise scarfers coming.
U-turn unstabbed isn't scaring much. This isn't scizor choice-banding a u-turn, you're still mainly a scouting u-turn.
If Flygon isn't even carrying the scarf, then I'm even less impressed, as now it's slightly better power (assuming choice band) with much less speed. I don't see the point to choice specs flygon at all in comparison, but that doesn't seem good either.
Also, you underestimate Boomburst. That's a 140 base power move that hits through subs. It doesn't need stab to hurt.